1. Field of the Invention
The present invention relates to a coating method and, more particularly, to a coating method in which a paint coated on a coating substrate is caused to flow or sag artificially by an outside action while rotating the substrate about its horizontal axis at least in one of a step of coating the substrate with the paint and a step of drying the paint coated on the substrate.
2. Description of Related Art
The subject matter of this application is related to the subject matter of commonly-assigned and copending application Ser. No. 07/100,767, now U.S. Pat. No. 4,874,639, of application Ser. No. 07/153,669, now abandoned, and of application Ser. No. 07/323,237, now U.S. Pat. No. 4,919,977.
Technology relating to the rotation of a coating substrate after coating is disclosed in Japanese Utility Model Publication (kokoku) No. 2,228/1976, Japanese Patent Publication (kokai) Nos. 30,581/1982 and 67,332/1973 as well as U.S. Pat. No. 4,092,953.
Other technology relating to coating a coating substrate is disclosed in Japanese Utility Model Publication (kokai) Nos. 80,930/1983, 109,430/1984, and 21,361/1975, Japanese Utility Model Publication (kokoku) No. 20,053/1981, Japanese Patent Publication (kokai) No. 4,471/1984, as well as U.S. Pat. Nos. 1,948,091, 2,658,008, and 2,598,163.
A coating method for coating an outer surface of a coating substrate such as a vehicle body generally includes a preparation step for preparing for the coating substrate to be coated with a part by removing dust from the substrate, a coating step for coating the coating substrate with the paint, and a drying step for drying the paint coated thereon. The drying step generally comprises setting and baking steps particularly when a thermosetting paint is used. The setting step is usually carried out prior to the baking step at a temperature which is lower than the ambient temperature during the baking step, for example, at room temperature or at the temperature ranging from 40.degree. C. to 60.degree. C. in order to volatilize a solvent slowly so as to cause no pinholes on the coating surface during the baking step which is usually carried out at approximately 140.degree. C.
The coating substrate is held at a given position on a conveyance means such as a carriage while being conveyed during the preparation, coating, and drying steps.
A degree of flatness or smoothness on the surface of a coating on the coating substrate is one of standards for evaluating a quality of the coat surface. The higher a degree of flatness the smaller a degree of irregularities on a coating surface, thus producing a better coating surface. It is well known that a thicker film thickness of a paint may give a higher degree of flatness on a coating surface. A paint sprayed on the surface of a coating may be said to sag if it is visually observed that the paint coated thereon flows and finds traces on the coating surfaces by 1 mm to 2 mm until it is cured in the drying step. It may be defined herein that sags of the paint occur if such traces exceed at least 2 mm when visually observed. In other words, a sagging limit thickness of a coating is a film thickness beyond the maximum film thickness in which the coating does not sag and the coating in its sagging limit thickness is caused to sag at least in the drying step if it is left as it is after spraying. On the contrary, a film thickness of the paint within its sagging limit thickness is a film thickness in which it does not sag in the drying step even if it is left as it was sprayed.
It is known that sags or drips of a paint coated on the substrate during the coating step are detrimental to the quality of the coating surface. Such sags or drips are caused when the paint coated thereon flow downwardly due to gravity. They become more likely to occur if the paint is coated in a film thickness which becomes thicker in one coating.
And it is a matter of course that the sags are caused more likely on a coated surface extending in an up-and-downward direction than on a coated surface extending in a horizontal direction. This enables the paint to be coated on the surface extending horizontally in a film thickness which is thicker than on the surface extending in an up-and-downward direction because the sags or drips of the paint little affect adversely the coating surface extending in a horizontal direction. If the film thickness of a coat on the horizontally extending surface is the same as that on the surface extending in an up-and-downward direction, the former can produce a degree of flatness which is higher than the latter because the paint coated on the horizontally extending surface of a coating surface becomes flattened due to a natural flow in the paint to an extent to which no sags substantially occur.
Conventionally, in order to provide a coat with a higher degree of flatness while preventing sags or drips of a paint coated on the surface of a coating substrate, there have been used paints which are lower in viscosity and less flowable. Even if such thermosetting paints are used, however, a sagging limit of the paint coated on the surface extending in an up-and-downward direction is as high as approximately 40 .mu.m. This sagging limit thickness, referred to sometimes as a sagging threshold value, is the maximum film thickness in which the paint does not substantially sag on the surface of a coating substrate extending in an up-and-downward direction.
Sags of the paint are likely to occur at initial stages of the setting and baking steps, particularly at the initial stage of the baking step so that a film thickness of a coat is determined by a film thickness of the paint coated on the surface of a coating substrate to such an extent that the paint does not sag on an up-and-downwardly extending surface thereof. Accordingly, in order to provide a coating in a film thickness larger than a sagging limit thickness of the paint, the coating step is repeated twice or more in the conventional coating method.
It is effective to rotate a coating substrate about its horizontal axis extending in a substantially horizontal direction of the substrate in order to provide a coating with a higher degree of flatness. In other words, even if the film thicknesses are the same, a coating with a higher degree of flatness may be given by utilizing a flowability of the paint in such a manner that a direction in which gravity acts upon a surface of the coating is artificially changed by rotating the substrate. This technology may produce a similar effect even if the paint is coated in a film thickness which is thicker than a sagging limit thickness of the paint to the contrary of conventional coating techniques.